1. Field of the Invention
The present invention relates to a vapor-phase film growth apparatus, and more particularly to a vapor-phase thin-film growth apparatus for depositing, for example, a highly dielectric or ferroelectric thin film of barium titanate/strontium or the like on a substrate by way of vapor-phase growth. The present invention is also concerned with a gas ejection head for use in a vapor-phase thin-film growth apparatus, and more particularly with a gas ejection head capable of uniformly ejecting a material gas and an oxide gas.
2. Description of the Prior Art
The degree of integration of integrated circuits fabricated by the semiconductor industries has been increasing so rapidly in recent years that research and development activities are switching from present megabit-order DRAMs to gigabit-order DRAMs in the future. For manufacturing such DRAMs, it is necessary to use a capacitor element capable of achieving a large capacitance with a small area. Promising dielectric thin-film materials for use in the manufacture of such a capacitor element are metal-oxide thin-film materials including tantalum pentoxide (Ta.sub.2 O.sub.5) having a dielectric constant of about 20, barium titanate (BaTiO.sub.3) having a dielectric constant of about 300, strontium titanate (SrTiO.sub.3), and a mixture thereof, i.e., barium strontium titanate, rather than silicon oxide or silicon nitride having a dielectric constant of 10 or less.
For depositing a thin film of such a metal oxide on a substrate by way of vapor-phase growth, a material gas comprising one or more organic metal compounds and an oxide gas are mixed with each other, and the mixture gas is ejected toward the substrate which has been heated to a certain temperature.
More specifically, the substrate is delivered into a film deposition chamber (reaction chamber) and placed on a substrate holder. While the substrate on the substrate holder is being heated to the desired temperature by a heater that is either housed in the substrate holder or positioned below the substrate holder, the mixture gas is ejected toward the substrate from a gas ejection head disposed in the film deposition chamber.
When the thin film is grown on the substrate using the material gas, a metal compound such as SrO.sub.2, SrCO.sub.3, BaO.sub.2, or BaCO.sub.3, or an intermediate compound formed from the organic metal compound or compounds is generated and deposited on a mixture gas ejection surface of the gas ejection head. The deposited compound is then peeled of f from the mixture gas ejection surface due to the difference between the coefficients of thermal expansion of the deposited compound and the gas ejection head in heat cycles produced at the time the vapor-phase thin-film growth apparatus is operated and shut off. The peeled-off deposited compound tends to produce particles and contaminate the substrate.
To solve the above problem, it has been proposed to provide a cooling mechanism in the gas ejection head for circulating a coolant in the gas ejection head to cool the gas ejection head for thereby preventing metal compounds from being deposited on the gas ejection head.
However, the cooling mechanism is unable to completely prevent metal compounds from being deposited on the gas ejection head because the surface of the gas ejection head which faces the substrate is directly exposed to radiant heat from the substrate heated by the heater. Thus, there is a considerable difference between the temperature at which the mixture gas is kept and the temperature at which the thin film is deposited on the substrate.
Once a compound is deposited on the gas ejection head, it is necessary to remove the deposited compound from the gas ejection head. Hence, the gas ejection head needs to be taken out of the film deposition chamber. However, the process of cleaning the gas ejection head is quite difficult to carry out since gas supply lines for supplying the material gas and the oxide gas are connected to the gas ejection head, a coolant line is connected to the cooling mechanism, and the gas ejection head is heavy itself.
Because the organic metal compound gas is liquid at normal temperature, the gas ejection head needs to be heated in order to keep the gas from being condensed. According to one proposal, a heating liquid medium is introduced into the gas ejection head to heat the gas ejection head. FIG. 1 of the accompanying drawings shows a conventional gas ejection head which has a heating structure for heating the gas ejection head with a heating liquid medium, as disclosed in Japanese patent application No. 7-119260.
As shown in FIG. 1, the conventional gas ejection head comprises a lower plate 101 and an upper plate 102 which are spaced from each other, defining a hollow space therebetween. The upper plate 102 has a plurality of ribs 103 extending downwardly through the hollow space toward the lower plate 101 and dividing the hollow space into a circulatory passage 104 for a heating liquid medium. A plurality of nozzle tubes 105 are fixed, as by welding, to the upper plate 102 and the lower plate 101 and extend through the hollow space. While the heating liquid medium is flowing through the circulatory passage 104 to heat the gas ejection head, the material gas is ejected from the nozzle tubes 105 into the film deposition chamber.
Since the circulatory passage 104 lies fully over the mixture gas ejection surface of the gas ejection head, it provides a uniform heating capability to the entire mixture gas ejection surface. However, the process of manufacturing the gas ejection head is tedious and time-consuming because it is necessary to weld or otherwise fix each of the nozzle tubes 105 to the upper and lower plates 102, 101 for fully sealing the circulatory passage 104 from the nozzle tubes 105. Furthermore, many nozzle tubes 105 need to be fixed to the upper and lower plates 102, 101 in order to eject the material gas as uniformly as possible from the mixture gas ejection surface of the gas ejection head. Accordingly, the process of fixing the nozzle tubes 105 becomes more complex and time-consuming as the mixture gas is to be ejected more uniformly from the gas ejection head.
The material gas and the oxide gas need to be mixed at a proper position or time. If the material gas and the oxide gas are not mixed at a proper position or time, then various problems are caused.
If the material gas and the oxide gas are mixed too early, e.g., if the material gas and the oxide gas are mixed too far upstream of the gas ejection head, then the material gas will react before reaching the gas ejection head, and a substance produced by the reaction will be deposited on wall surfaces of the gas ejection head, possibly producing particles. The reaction of the material gas prior to the gas ejection head results in a reduction in the ability of the mixture gas to form a film on the substrate.
If the material gas and the oxide gas are mixed too late, e.g., if the material gas and the oxide gas are mixed downstream of the gas ejection head, then the position where they are mixed is too close to the substrate in the film deposition chamber for the material gas and the oxide gas to be well mixed together. The incomplete mixing results in a reduction in the ability of the mixture gas to form a film on the substrate.
The gas ejection heads used in a conventional vapor-phase film growth apparatus incorporate various structural details for uniformly distributing a gas or uniformly mixing a plurality of gases. For uniform distribution of a gas, for example, a resistance to gas flows is added to individual divided gas passages, or a gas passage is progressively branched into a plurality of smaller gas passages. For uniformly mixing a plurality of gases, for example, the gas ejection heads include design considerations given to the position where the gases are mixed together.
Because of those structural details, the gas ejection heads are highly complex in structure, and hence suffer the following shortcomings:
(1) The structure for heating the gas ejection heads is complex. PA1 (2) The material gas tends to be condensed and decomposed in the gas ejection heads, producing deposits in the gas ejection heads. PA1 (3) It is not easy to clean the gas ejection heads to remove deposits. PA1 (4) The maintenance of the gas ejection heads is difficult to carry out. PA1 (5) The gas ejection heads which are structurally complex are difficult to manufacture. PA1 k: ratio of specific heats (constant dependent on the gas); PA1 p.sub.1 : the pressure (pa) of the gas at the gas inlet side of the orifice; and PA1 v.sub.1 : the specific volume (m.sup.3 /kg) of the gas at the gas inlet side of the orifice.